Gas Pipelines and Compressors are responsible for "The mysterious hum" that is being heard around the world.

Why "the Hum" gets louder at night ?
One of the mysteries surrounding "The Hum" is why does it seem to get louder or be more prevalent at night or in the wee hours of the morning This is a question I have had difficultly resolving until I stumbled upon this "Reccomended for you" Post One of the Comments describes a process call "Line Packing" that is employed with natural gas pipelines.

Without a doubt this explains why the Hum is louder at times and particularly louder at night.

What is Linepack ?
 

"Linepack" refers to the volume of gas that can be "stored" in a gas pipeline. Thinking back to basic chemistry, gas can be compressed (unlike liquids). Think of the classic example of compressing air in a bicycle pump. The air can be compressed into a smaller volume, or more air could also be squeezed into a fixed volume - for example a tyre.

The operational implications of line pack mean that the volume of gas injected into a pipeline (at the inlet), can bd greater than the volume of gas withdrawn from the pipeline (at the outlet). This frequently occurs due to the unpredictable nature of end-user operations and hence, their gas demand.

However, when gas is 'stored' in the pipeline by compressing it, the pressure exerted on all parts of the pipeline increases. The quantity of additional gas volume that can be stored in a pipeline depends on the pressure rating of the pipe, flanges, non return valves, compressors etc, as well as the ability for equipment upstream (before the inlet) and downstream (past the outlet) to respond to a sudden surge in pressure if inlet or outlet valves failure. This event is called a high-pressure/low-pressure breakthrough. When the pipeline pressure is high, it becomes increasingly difficult to inject additional gas into it. Compressors are used to increase the injection pressure of the gas.